A digital modulator-demodulator or "modem" is a data communications device which allows communication between digital computers or other digital data devices over voice-grade telephone lines. The modem converts digital data signals from the computer or device into a form of analog signal, discrete in time value, that is suitable for transmission over analog voice band circuits, and vice versa for incoming data signals.
Intelligent modems are available which perform functions in addition to the modulation and demodulation of data. For example, intelligent modems will accept coded signals from the user to control certain aspects of the modem's transmission or reception parameters. These coded signals comprise a standard command set such as that known as the "AT" command set, for example. An exemplary AT command is given by the instruction characters "ATSO=4" which is used to enable the automatic answer feature of the modem and select the number (in this instance four) of rings to be counted before the modem answers the telephone line. In conventional intelligent modems the coded signals are provided to the modem in what is referred to as a "command mode." When the modem is operating in the command mode, the coded signals are not transmitted over the telephone line, but are implemented internally by the modem. The command mode is distinguished from a "transmission mode" during which data transmission occurs over the telephone line. The transmission mode is also sometimes referred to as the "on-line" mode or the "transparent" mode. When the modem is operating in the transmission mode a mechanism is provided for switching the operation of the modem to the command mode for implementing command instructions, it being understood that such occurs without loss of the communications link over the telephone line. A known mode switching mechanism is to issue a predetermined sequence of escape commands referred to as an "escape sequence" during the transmission mode. In order to avoid an unintended transition into the command mode, the escape sequence is typically preceded or bounded by a silent interval of a predetermined time duration in which no data is being transmitted on the line.
The foregoing conventional mode switching mechanism has certain disadvantages. Because the escape sequence is entered during the transmission mode, the escape sequence data is unnecessarily transmitted on the telephone line to the receiving device. This results in the receipt of spurious information by the receiving device and may also initiate potentially damaging commands in the receiving device. Further, at least two separate instructions are required in order to effect a transition from the transmission mode to the command mode and execute a command.